for($data as $key=>$value){
   print_r($value);
}

'Hello. I have two dots. Please replace them!'

'Hello_ I have two dots_ Please replace them!'

$text='Hello. I have two dots. Please replace them!';

echo IHateStrReplace(".","_",$text);

function IHateStrReplace($replace_from,$replace_to,$input)
{
    $result="";

    for($i=0;$i<strlen($input);$i++)
    {
        $result.= ($input[$i]==$replace_from)?$replace_to:$input[$i];
    }

    return $result;
}

$var = 'Hello. I have two dots. Please replace them!';
echo preg_replace('#.#', '_', $var);

foreach($data as $key => $value){
    $data[$key] = preg_replace('/./', '_', $value);
}

print_r($data);

$original_string = 'Hello. I have two dots. Please replace them!';
$exploded_string = explode('.' , $original_string);
$new_string = implode('_' , $exploded_string);